1. Field of the Invention
This invention relates, in general, to hard-copy printers and, more specifically, to data storage systems for gray-scale printers capable of accepting different types of input data.
2. Description of the Prior Art
Hard-copy printers, whether of the electrophotographic, impact, ink jet, or other type, are becoming more versatile and many now have the capability of printing pictures, or pictorial information, as well as text, graphics, and like images. With a high level page description language, such as PostScript which is a registered trademark of Adobe Systems Incorporated, it is possible to specify the location of a pictorial image on the page of the printed hard-copy. In some instances, it is also possible to select a portion of a picture for enlargement or reduction and inclusion on the page. As a general rule, these pictures are input from electronic data originating from video sources such as video cameras, video tape recorders, or picture scanners.
There is one basic difficulty which presents a problem when merging text and graphics from one source of data with pictorial information from another source of data. The text information must be interpreted from the page description language and loaded into a page memory system in order that it can be reproduced on a printed page by the printer electronics. While some devices have less than a full page of memory dedicated to the printing buffer, there are certain advantages to a complete page buffer in electronic hard-copy printers. The size of the page buffer is dependent upon several factors, including the size of the page, the resolution desired, the number of colors to be printed, and the level of color graduations, or the gray-scale increment. While the capacity of the page buffer can be significant, it is well within the state of the art to provide such capacity for text and graphics information in a page buffer.
Storing video or pictorial information in a page buffer presents the bigger challenge. Pictorial data contains much more information than text information and thus requires more memory, or bits, to sufficiently store the data. When placing the picture in the same page buffer which is used to store the text for the page, a very large amount of memory must be used. Text can adequately be stored at a relatively low bit level per pixel, with one to three bits usually being sufficient. This is primarily because a wide range of colors and gray-levels are not needed to faithfully reproduce the desired text and most graphics information. On the other hand, pictures often need four or more bits per pixel for each color to adequately reproduce the picture content. When translating this into the page buffer, the higher bit level requirements of the pictorial data uses a considerable amount of memory space.
In addition to the memory requirements necessitated merely because of the higher bit requirements of pictorial information, the utilization of a page buffer to store pictorial information is also inefficient because the resolution of the video source is not usually sufficient to put meaningful data in all of the pixels of the page buffer delegated to the picture. This is especially true when the picture is enlarged for insertion into the page. The original video can have a resolution of about 512 horizontal and vertical pixels for the entire picture. Since the page buffer can have much higher resolution, such as 400 dpi (dots per inch), positioning the picture in an area of the page bounded on each side by 2,000 pixels (5 inches) means that about one-sixteenth of the memory in the page buffer will be representing actual pictorial information and the other memory locations will contain processed information from the original video signal.
The problems associated with storing and combining text/graphics information with pictorial information are addressed by several issued patents. U.S. Pat. No. 4,553,172, issued on Nov. 12, 1985, discloses a system whereby picture and character information is separately processed and converted into digital values at different bit levels. The color pictures use 32 bits, and the characters use 25 bits. The large number of bits for the characters is produced by combining several character data words or signals together to form a combined data word which is similar in size to the color picture data word. Both types of data are stored and recalled from memory as 32-bit words. While separate processing of text and pictorial information is shown, the storage system in this patent is much different than that of the present invention.
U.S. Pat. No. 4,618,846, issued on Oct. 21, 1986, discloses a data coding system. Column 1 of the patent recognizes that three or four bits per color are necessary to represent pictorial information, and that fewer levels, even one bit, can be used to represent text on a newspaper page. While recognizing the required bit levels for representing different types of data, the data coding system of the patent is not similar to the teachings of the present invention.
U.S. Pat. No. 4,626,902, issued on Dec. 2, 1986, provides some background information on the resolution needed to adequately represent "literal" text components and pictorial components. In this case, the pictorials are half-toned and may require finer pixel resolutions to properly represent the data for the picture.
Therefore, it is desirable and it is an object of this invention, to provide a printer having a memory system which makes efficient use of the memory space taking into consideration the requirements of the data being represented and the intermediate and ultimate resolution which must be maintained to preserve data quality.